1. Technical Field
The invention relates generally to integrated circuitry; and, more particularly, it relates to systems and methods that employ fully differential voltage controlled oscillators.
2. Related Art
Prior art voltage controlled oscillators (VCOs) are commonly implemented using NMOS (n channel metal oxide semiconductor) transistors or a combination of NMOS and PMOS (p channel metal oxide semiconductor) transistors. In many integrated circuit applications, a VCO is one of the main high-speed components in a device. As a result, the noise generated by the VCO is a major factor that determines the overall performance of the device. In addition, the noise immunity of a VCO against the noise from a power supply and the substrate may have a very large impact on the robustness of the device when operating in a noisy environment.
For example, the VCO is a common building block in phase locked loops (PLLs); the VCO is used to provide a very clean local frequency reference. In such application, the PLL often needs to be very well shielded from noise sources. The noise pickup of a VCO scales with its size; the larger the size of the VCO, then the more antenna-like the circuit will act in terms of noise pick up. In addition, VCO applications commonly operate at very high frequencies. These two competing needs of a VCO (to operate at very high operating frequencies and to generate relatively low noise) are intimately coupled in design efforts to achieve a high performance PLL.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings.
Various aspects of the invention can be found in an all PMOS (p channel metal oxide semiconductor) fully differential voltage controlled oscillator (VCO). The differential implementation of the PMOS VCO enables the rejection and elimination of a large degree of noisexe2x80x94especially common mode noise. All of the active devices may be implemented in an NWELL that is isolated from the remainder of the substrate. Therefore, the VCO will effectively see less substrate noise and will be less susceptible to substrate coupling. PMOS transistors typically have mush less 1/f noise compared to NMOS transistors. The 1/f noise is undesirable because it can be up-converted to the operating frequency of the device and thereby degrade the noise performance of a VCO. The present invention is operable using only PMOS transistors and thus eliminates one of the most dominant noise sources of the VCO.
In addition, the present invention obviates the need for a deep NWELL that is commonly required for VCO designs that use a combination of NMOS and PMOS transistors to reduce substrate coupling. Implementing a deep NWELL requires proper biasing and a need for separation from other circuits by a certain distance. The present invention does not require the deep NWELL associated in NMOS implementations, and it thereby simplifies the design layout and thus allows for a more compact design. This improved compacted-ness provides for less parasitic capacitance from interconnects. This makes it much easier to have the VCO frequency to be closer to that of the actual silicon. In addition, less parasitic capacitance means that a VCO designed in accordance with the present invention provides for a greater tuning range when compared to prior art VCOs. Because the physical layout of the VCO can be smaller, it is also less susceptible to the noise coupled from substrate.
The present invention also employs a center-tapped inductor and differential tuned varactor. The center-tapped inductor is laid out directly on the substrate in certain embodiments. This fully differential implementation, offered by the present invention, fits well into the concept of a fully differential PLL design. The center tap of the center-tapped inductor (the AC ground) is shunted to the device""s DC ground. A resistor is employed in certain embodiments. In doing this, the VCO is enabled to function with a power supply higher than the maximum voltage drop that a device can sustain as determined by the specific device""s processing. The VCO can be effectively over-driven to provide for higher gains as desired. Additional components may be used in shunting the inductor""s center tap to ground, including capacitors. By changing the value of the shunt to ground devices, the amplitude of the oscillation and the common mode of the output signal may be adjusted. Because the frequency of the VCO is not sensitive to the capacitance of the AC ground, a large capacitance may be connected from the center-tapped inductor""s center tap to a clean power supply to filter noise from the substrate even further.
The above-referenced description of the summary of the invention describes some of the various aspects of the present invention. The claims are also directed to some other of the various other embodiments of the subject matter towards which the present invention is directed. In addition, other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.